* Astronomy

Emission nebula IC 1805



The Heart Nebula, IC 1805, Sh2-190, lies some 7500 light years away from Earth and is located in the Perseus arm of the Galaxy in the constellation Cassiopeia. This is an emission nebula showing glowing gas and darker dust lanes. The nebula is formed by plasma of ionised hydrogen and free electrons.
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Astronomers announced today that they have found a nebula. Known as W3, this glowing gas cloud 7000 light-years away in the constellation Cassiopeia has just begun to shine with newborn stars. Shrouds of dust currently hide its light, but this is only a temporary state. In 100,000 years - a blink of the eye in astronomical terms - it may blaze forth, delighting stargazers around the world and becoming the Grand Nebula in Cassiopeia..

"The Grand Nebula in Cassiopeia will appear in our sky just as the Great Nebula in Orion fades away. Even better, its home constellation is visible year-round from much of the northern hemisphere" - Tom Megeath, Smithsonian astronomer (Harvard-Smithsonian Centre for Astrophysics), who made the announcement in a press conference at the 207th meeting of the American Astronomical Society.

The Orion Nebula is one of the most famous and easily viewed deep-sky sights. It holds special significance for researchers as the nearest region of massive star formation.
The star formation process begins in a dark cloud of cold gas, where small lumps of material begin to contract. Gravity draws the gas into hot condensations that ignite and become stars. The most massive stars produce hot winds and intense light that blast away the surrounding cloud. But during the process of destruction, stellar radiation lights up the cloud, creating a bright nebula for stargazers to admire.

"Orion may seem very peaceful on a cold winter night, but in reality it holds very massive, luminous stars that are destroying the dusty gas cloud from which they formed. Eventually, the cloud of material will disperse and the Orion Nebula will fade from our sky" - Tom Megeath.


Position (2000): RA: 3h01m56s Dec: +60d35m50s
This artist's depiction simulates the view that amateur astronomers on Earth might enjoy 100,000 years from now, when the Grand Nebula in Cassiopeia lights up.
Credit: David A. Aguilar

Of special interest to Megeath is a system of four bright, massive stars at the centre of Orion known as the Trapezium. These stars bathe the entire nebula with powerful ultraviolet radiation, lighting up nearby gas. Even a modest telescope reveals the Trapezium surrounded by billowing ripples of matter gleaming eerily across the vastness of space. Yet the Trapezium is only the tip of the iceberg, surrounded by more than 1000 faint, low-mass stars similar to the Sun.

"The question we want to answer is: why are these massive stars sitting in the centre of the cluster?" - Tom Megeath.

There are two competing theories to explain the Trapezium's location. One holds that the Trapezium stars formed apart from each other but descended to the centre of the cluster, ejecting a spray of low-mass stars in the process. The other leading theory is that the Trapezium stars formed together in the centre of the cluster and have not moved from their birthplace.

"Obviously, we can't go back in time and look at the Trapezium when it was still forming, so we try to find younger examples in the sky" - Tom Megeath.

W5 is also known as IC1848 and together with IC1805 it is part of a complex region popularly dubbed the Heart and Soul Nebulae.
Such proto-Trapeziums would still be buried in their birth cocoons, hidden to visible-light telescopes but detectable by radio and infrared telescopes. Searches at those longer wavelengths have identified many regions where massive stars are forming, but could not determine whether the protostars were alone or in collections of four or more stars that could be considered Trapeziums.
Megeath and his colleagues examined one such protostellar clump in W3 using the NICMOS instrument on the Hubble Space Telescope and the National Science Foundation's Very Large Array. They discovered that the object, which was thought to be a binary star, actually contained four or five young, massive protostars, making it a likely proto-Trapezium.
These protostars are so young that they appear to be still growing by accreting gas from the surrounding cloud. All of the stars crowd into a small area only about 500 billion miles across (just under one-tenth of a light-year), making this cluster more than 100,000 times denser than stars in the Sun's neighbourhood. This suggests that the massive stars in Orion's Trapezium formed together in the centre of the cluster.
The same physical processes that have carved the Orion Nebula now are moulding the W3 nebula. The massive stars in this compact group are starting to eat away at the surrounding gas with ultraviolet radiation and fast stellar outflows. Eventually, they will destroy their dense cocoon and emerge to form a new Trapezium in the centre of W3. However, the final form of the nebula and the time that it will reach maximum brilliance are uncertain.

"Who knows, in 100,000 years the emerging Grand Nebula in Cassiopeia may replace the fading Orion Nebula as a favourite object for amateur astronomers. In the meantime, I think it will be a favourite target for professional astronomers trying to solve the riddle of massive star formation" - Tom Megeath.

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